On board Atlantis is a metal girder section that the crew is carrying that is designed to support tens of tons of equipment, solar panels and other systems necessary to survival in orbit should measure more than 100 meters (109 yards) after its completion in 2004.

The crew is to spend the bulk of its mission assembling the Mobile Transporter, rail system to carry the station's Canadian-built mechanical arm along a 91-meter (300-foot) truss when it is completed in 2004.

The structure -- the longest ever assembled in space -- will move the arm at a speed of just 91 meters (300 feet) per hour as the station travels around the Earth at 27,000 kilometers an hour (17,000 miles an hour).

"The countdown is continuing," said NASA test director Pete Nickolenko at the Kennedy Space Center here. "All our flight and ground systems are looking great. The team is ready and looking forward" to the trip back to the station, he said.

Originally NASA had tried to keep the exact launch time a secret until 24 hours before launch as a security measure against possible terrorist threats. The space agency instead issued only a general four launch warning notice that said launch was scheduled for sometime between 2:00 pm (1900 GMT) and 6:00 pm (2300 GMT) Thursday.

However, anyone with goods math and the orbital data for the space station would be able to estimate the launch time to within a few minutes. Moreover, several contractors had already released the launch time as part of other pre launch preparations forcing NASA to belatedly officially announce the launch time late Wednesday. NASA, says no specific terrorist threat has been reported.

Shuttle commander Michael Bloomfield, co-pilot Stephen Frick and mission specialists Rex Walheim, Ellen Ochoa, Lee Morin, Jerry Ross and Steven Smith, will transport and install the railcar and first Mobile Transporter truss segment during the mission, the 13th to the station.

Atlantis is scheduled to return April 15.

The first truss section, a 13-meter girder weighing 13 tonnes, was build by Boeing at a cost of 600 million dollars.

Late Tuesday, NASA shuttle program director Ron Dittemore mentioned that he had a slight concern regarding the external tank door power drive unit, which closes two fuel trap doors when the shuttle separates from its external tank. About eight minutes after blastoff the shuttle jettisons the tank, which then falls and is destroyed upon re-entering the atmosphere.

Tests carried out on the shuttle Endeavour showed these trap doors are closing slower than usual -- 54 seconds instead of 48 seconds.

NASA engineers Wednesday thoroughly tested the closing system to make sure Atlantis shuttle can be safely launched, with Dittemore declaring NASA was "very optimistic" about the outcome.

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